The invention relates to a device for optical determination of ingredients of a pourable product, in which the product, as a stream in a conduit, passes a measurement window, in particular for analysis by means of a spectrometer with a predetermined measurement duration for performing the analysis.
The invention also relates to a method for near-time optical determination of ingredients in the stream of a pourable product, preferably by spectroscopic processes, in particular NIR spectroscopy.
Such devices and methods are used for presentation of pourable product, also called bulk goods, for optical analysis, for instance by means of colorimetric processes but in particular also for spectroscopic optical analysis. Replicable presentation which has the least possible variance in the result for identically composed bulk products, presents problems especially for poorly flowing powdered bulk goods with very small particle size. Particular attention must therefore be dedicated to preparing the specimens. This can be assured in the laboratory by suitable manual handling of the specimens. Problems often arise, however, in near-time monitoring of continuous processes, that is, on-line, where the corrective influence of the human being is lacking. Spectroscopic monitoring of liquids is the state of the art. Although bulk goods are also flowable, nevertheless, unlike liquids, they exhibit a very widely scattered rheological behavior, depending on the existing environmental conditions. For instance, the internal friction in such pourable products varies as a consequence of humidity and temperature, particle size, relative density, particle shape, process control, and so forth. The electrical properties also play a role, since the particles stick together more strongly with a static charge.
In particular, a steady exchange of material at the measurement window is necessary so that the results of measurement will now be adulterated. Electrostatic charges can make the exchange of material still more difficult.
Compared with laboratory measurement, on-line measurement presents some additional problems, which unless they are solved make accurate measurement impossible. The product must be fed continuously to the optics, yet during the individual measurement cycles, lasting up to about 20 seconds each, the product must not be moved. Furthermore, it must be assured that the product is thoroughly mixed. Finally, the correct, replicable product consolidation has substantial influence.
In measurements, the tendency of bulk goods to partly stick to the measurement window presents very particular problems, because this additionally adulterates the result of measurement. In the prior art, such problems are solved in various ways.
To solve these problems, European Patent Disclosure EP 0 585 691 contemplates fluidizing the powder; that is, by blowing gases in, it is converted into a fluidized bed into which the measurement window of a spectroscopic analysis system plunges.
In another apparatus in accordance with International Patent Disclosure WO 95/24633, the stream of product is carried via a vibrating feed channel. The vibrating feed channel feeds the product horizontally. In the boundary walls of the vibrating feed channel, a measurement window is provided, through which the spectroscopic analysis is performed.
A measurement system of the present Applicant, known by the tradename InfraAlyzer 600, is constructed in a similar way. In it, an optical module with the measurement window is disposed above the vibrating feed channel. The vibrating feed channel serves to furnish the most constant possible layer thickness of the stream of bulk goods. Disposing the measurement window above the vibrating channel prevents soiling of the window.
Finally, British Patent GB 2 142 721 discloses an apparatus for measuring powdered material in which the material is moved past a measurement window in a conduit with the aid of a worm conveyor. During the measurement, feeding is interrupted.
A disadvantage of the known apparatuses is their relatively complicated construction, which requires major expense for apparatus and engineering. Furthermore, constant operating conditions can be assured only with difficulty.